Effect of thickening outermost layers in Al/Ni multilayer film on thermal resistance of reactively bonded solder joints

Abstract

To reduce the thermal resistance of solder joints obtained by Al/Ni self-propagating exothermic reaction, we focus on the interface between the solder and the reactive Al/Ni multilayer before the reaction. We fabricate an Al/Ni film in which the outermost Al or Ni layer is deposited more thickly. By the laser flash method with response function analysis, the thermal resistance of the bonded specimens was measured in the case of thickening and non-thickening of the outermost layer. It is found that the resistance decreases with the thickness of the outermost layers, particularly the Ni layer. To investigate the influence of the amount of void on the resistance, scanning acoustic tomography (SAT), scanning electron microscope (SEM) observation, and field-emission electron probe micro-analysis (FE-EPMA) were carried out. Differences in the interfacial state between Ni-thickening and Al-thickening specimens are discussed in light of solder wettability, diffusion into the solder, and the type of intermetallic compound at the solder interface. It can be considered that thickening the outermost Ni layer has a great impact on decreasing the number of voids generated at the AlNi–solder interface even in low-pressure bonding, leading to reduction in the thermal resistance of solder joints.